1. Field of the Invention
The present invention relates to a radio frequency coil assembly for collecting a magnetic resonance signal from a tested person based on a magnetic resonance phenomenon and a magnetic resonance imaging (MRI) apparatus having the radio frequency coil assembly.
2. Description of the Related Art
Magnetic resonance imaging (MRI) performed by an MRI apparatus is an image pickup method of magnetically exciting nuclear spins of a tested person under a static magnetic field with a radio frequency signal having a Larmor frequency to generate a magnetic resonance (MR) signal and reconstructing an image from the generated MR signal.
In order to implement the image pickup method, the MRI apparatus includes a static magnetic field magnet for generating a static magnetic field and a mechanism for applying a gradient magnetic field pulse and a radio frequency magnetic field pulse to the tested person according to a predetermined pulse sequence. The gradient magnetic field pulse is transmitted to the tested person through a gradient magnetic coil which is disposed in a bore of the static magnetic field magnet and connected to a gradient magnetic field power source. Similarly, the radio frequency magnetic field pulse is transmitted to the tested person through a transmission radio frequency coil which is disposed in the bore of the static magnetic field magnet and connected to a transmitter. In order to receive a magnetic resonance signal including a radio frequency signal generated from the tested person, a reception radio frequency coil is disposed in the vicinity of the tested person. Although a single coil may be used as the transmission and reception radio frequency coils, in many cases, dedicated reception radio frequency coils are used according to diagnosis positions.
For example, in order to obtain an image with a high sensitivity, a plurality of surface coils (an array coil) as a reception radio frequency coil are disposed in a diagnosis region of the tested person, and an image thereof is picked up. For example, as a backbone coil, an array coil where QD surface coils are arranged in a body axis direction is disclosed in JPA H5-261081. The array coil is shown in FIG. 30.
Now, a QD surface coil will be described. As shown in FIG. 31, a QD coil 120 is a coil constructed by overlapping a loop-shaped surface coil 121 and an 8-shaped surface coil 122. Since a total sum of radio frequency magnetic fields generated in a loop is zero, the coils can be disposed to overlap with each other in a magnetically decoupled state. When the radio frequency magnetic fields B1 generated from two coils 121 and 122 in a cross section taken in line A-A′ are seen, the radio frequency magnetic fields B1 are perpendicular to each other in the axis as shown in FIG. 32. In this case, since noises from the coils 121 and 122 are independent of each other, in a state that the signals are shifted by 90° from each other, if the signals are added to each other, SNR is as follows.SNR∝|B1(loop-shaped)|+|B1(8-shaped)|)/√2
FIG. 33 shows characteristics of SNRs in the axes of the coils 121 and 122. A long dotted line shows an SNR profile of the loop-shaped surface coil 121, and a short dotted line shows a profile of the 8-shaped surface coil 122. A solid line shows an SNR profile of the QD surface coil 120 obtained by perform 90°-shifting and addition processes. It can be seen that the SNR of the QD surface coil 120 is 21/2 times higher than that of a position where the SNRs of the loop-shaped surface coil 121 and the 8-shaped surface coil 122 are equal to each other. In addition, it can be seen that the SNR of the QD surface coil 120 is higher than those of the two coils 121 and 122 in a wide range. Like this, a high SNR can be obtained by using the QD surface coil 120 in comparison to a case where the loop-shaped surface coil or the 8-shaped surface coil is individually used.
On the other hand, as a case where an image of the entire abdomen is picked up, a method of receiving a signal from the entire abdomen by using a plurality of surface coils which are disposed to surround the tested person is disclosed in JPA 2003-334177. As shown in FIG. 34, in many cases, an array coil constructed by arranging a plurality of loop coils corresponding to a body surface is used as the surface coils.
Like this, by disposing a plurality of the surface coils corresponding to the imaged portions, it is possible to obtain an image for the imaged portions with the highest sensitivities thereof. However, since there is a need to allocate coils corresponding to the imaged portions, the number of coils increases, and the coils need to be changed according to the imaged portions when the tested persons are changed. Accordingly, a large number of coils must be prepared, and the task of changing the coils is burdensome to medical technicians or doctors.
Like this, in the conventional reception radio frequency coil, since different dedicated array structures according to the imaged portions are used, the operators (medical technicians or doctors) must change the array coils when the imaged portion is changed. The changing task is burdensome to the operator, and much time is taken for the operator to perform the task. Therefore, preparation burden to the operator increases, and the task is one of the major factors of deterioration in patient throughput.
Recently, a technique of increasing the SNR of the QD surface coil by disposing a plurality of loop coils so as to be decoupled from each other and overlapping 8-shaped coils which intersect a central loop coil in an 8-shaped manner has been developed. An array coil is constructed by arranging a plurality of the coil sets in a direction perpendicular to an array direction of the loop coil, and the array coil is disposed on a top board, so that an image of the backbone of the tested person is picked up.
Although there is a difference between individual tested persons, when a tested person lies on the top board, in many cases, the backbone may be in a position relatively far (deep) from the top board, that is, the array coil. For example, the position may be 10 cm far from the top board. In this case, if the 8-shaped coil is disposed to overlap with only the aforementioned central loop coil, the sensitivity of collection of the signal from the relatively deep backbone is insufficient, and the SNR thereof is too low.
On the other hand, in the backbone coil shown in FIG. 35, four QD surface coils QD1, QD2, QD3, and QD4 are arranged in the body axis direction. In the technique, although the magnetic resonance signals emitting from many portions of the long backbone in the body axis direction are received by a plurality of corresponding surface coils so as to increase a range of the image-picked-up region, the sensitivity of a localized imaging process is not greatly improved.
In addition, a technique of alternately disposing coil units having four equivalent surface coils arranged in a direction intersecting the body axis direction may be used. By doing so, the signals emitted from localized portions of the backbone can be received by the four surface coils, so that it is possible to improve the sensitivity of the localized imaging process.
However, since the four equivalent surface coils are arranged in the direction intersecting the body axis direction, the outer surface coil is too far from the backbone, so that it is difficult for the outer surface coil to obtain a sufficient sensitivity. In other words, if the four surface coils are provided, there is a problem in that the corresponding sensitivity of the imaging process may not be sufficiently improved.